1. Field of the Invention
The present invention relates generally to an optical disc apparatus like a digital versatile disc (DVD) recorder and, more particularly, to a technique for automatically adjusting focus balance during recording and reproduction of information on an optical disc.
2. Description of the Related Art
An optical disc apparatus is so configured as to project laser light from an optical pickup onto an optical disc, produce a detection signal by picking up part of the laser light reflected from the optical disc, and perform focus control of the optical pickup based on the detection signal. The optical disc apparatus adjusts focus balance to ensure that focus control operation be executed under optimum conditions to achieve high accuracy recording and reproduction. The focus balance is a balance of an S-curve which represents a characteristic of a focus error signal produced from the reflected laser light from the optical disc. The focus balance can be varied by adjusting gain of a photodetection amplifier or by adjusting an offset value. As an example, focus balance adjustment is accomplished by measuring jitter of a reproduced signal and adjusting the focus balance to obtain a focus balance value at which the jitter is minimized, where the jitter is temporal variation of a reproduced clock signal from a reference clock signal.
Some previously known arrangements for performing focus balance adjustment are found in Japanese Laid-open Patent Application Nos. 1994-243496, 2001-307332 and 2005-216461, for example.
The arrangement disclosed in Japanese Laid-open Patent Application No. 1994-243496 is to generate a difference value signal representing differences between peak and bottom levels of a radio frequency (RF) signal, forcibly defocus a laser beam in two opposite directions (positive and negative directions) by a specified amount from a convergence point in a focus servo loop, and adjust levels of the difference value signal so that positive and negative levels of the difference value signal would be equalized to thereby accomplish the focus balance adjustment.
The arrangement proposed in Japanese Laid-open Patent Application No. 2001-307332 is to vary a focus balance value in specified steps to determine primary jitter amounts which are the amounts of jitter obtained at different focus balance values, calculate a secondary jitter amount by averaging the primary jitter amount for each focus balance value and the primary jitter amounts corresponding at least to values of an adjustment parameter immediately preceding and following the primary jitter amount, and adjust the focus balance value based on the secondary jitter amount thus calculated.
The arrangement described in Japanese Laid-open Patent Application No. 2005-216461 is to measure the amounts of jitter at different focus balance values while varying the focus balance value, measure tracking error signal values at different focus balance values while varying the focus balance value if a difference between maximum and minimum measured values of jitter is equal to or less than a specified value, and set the focus balance value at a value at which the tracking error signal value is maximized.
Typically, an optimum focus balance value does not remain constant all along a circumferential direction of an optical disc. Also, inner and outer tracks of the optical disc have different optimum focus balance values due to warpage of the disc or aberration of an optical pickup in many cases. In addition, the optimum focus balance value can vary due to tilt control operation which is performed to control tilting of the disc. Therefore, if the optical disc is poor in quality with great warpage, the focus balance value may deviate from the optimum value, making it impossible to correctly record and reproduce information on the disc, even when the tilt control operation is carried out.
Furthermore, even when an attempt is made to measure jitter level of a reproduced signal in real time and automatically adjust the focus balance value in accordance with variations in the jitter level, the optimum focus balance value can vary in different directions from one optical disc to another, and from one disc drive to another. The focus balance value may increase from the inner tracks to the outer tracks on one disc or decrease from the inner tracks to the outer tracks on another disc. There can also be a case where the focus balance value once increases and then decreases from the inner tracks to the outer tracks. It is therefore impossible to adjust the focus balance value to an optimum value based simply on jitter variations.
For reasons discussed above, conventional optical disc apparatuses have a problem that it is often impossible to automatically adjust the focus balance value to an optimum value all along the circumferential direction of an optical disc and to correctly record and reproduce information on low-quality discs having warpage. Nevertheless, none of conventional technologies including the arrangements of the aforementioned Japanese Laid-open Patent Applications provide a solution to this problem of the prior art.